1. Field of the Invention
The present invention relates to electric machines generally, and more specifically to permanent magnet synchronous machines.
2. Description of the Related Art
Electric motors are used in many applications. One type of electric motor, the permanent magnet synchronous motor, provides advantages for some of these applications. Permanent magnet synchronous motors can provide very high power density (that is, a large amount of power output per unit of physical size of the motor).
In a typical permanent magnet synchronous motor, a generally-cylindrical rotor comprises circumferentially-disposed permanent magnet poles. Opposing the rotor is a generally-cylindrical stator. The stator has teeth facing the permanent magnet poles. The teeth are wound with coils of wire such that supplying electric current to the coils magnetizes the teeth. When alternating current is applied to the coils, interaction between the permanent magnet rotor poles and the stator teeth motivates the rotor to rotate.
Magnetic circuits exist in a permanent magnet synchronous motor, the magnetic circuits including, among other things, the permanent magnet rotor poles, the stator teeth, and the air gap between the rotor and the stator. At any instant in the rotation of the rotor, those magnetic circuits will each have a reluctance. The reluctances of the various magnetic circuits change as, for example, a magnetic pole of the rotor transitions from being opposite a tooth of the stator to being opposite a gap between teeth.
In the use of a permanent magnet synchronous motor, reluctance torque or "cogging torque" can be a concern. Reluctance torque is generated due to changes in reluctance of magnetic circuits in the motor as the rotor rotates. When a change in reluctance occurs, a torque (unintended in the design of the motor) is produced. That torque is usually quite small with respect to the intended torque which the motor produces. However, that torque may be large enough to be disruptive in a number of applications for permanent magnet motors, such as electric power steering and electric suspensions for motor vehicles. In such applications, the reluctance torque may be enough to be felt by people in the motor vehicle.
Notching of the teeth of a permanent magnet synchronous motor has been used in an attempt to reduce reluctance torque. For example, U.S. Pat. No. 5,028,073, issued to Harms et al., discloses a permanent magnet motor with notched stator teeth. Although the design disclosed in the patent provides some benefit, further reductions in reluctance torque are still necessary for some applications of permanent magnet synchronous motors.
Therefore, a permanent magnet synchronous motor which can further reduce reluctance torque will provide advantages over the prior art.